1. Field of the Invention
The present invention relates to an electron microscope and also to a method of adjusting a monochromator.
2. Description of Related Art
Monochromators are used to achieve higher resolution in electron energy loss spectroscopy (EELS) implemented in transmission electron microscopes. Furthermore, monochromators are used to reduce chromatic aberration such that a maximum amount of information carried by a transmission image is increased. In addition, monochromators are used to improve the resolution by reducing chromatic aberration in a scanning electron microscope when a monochromatized electron beam with a low accelerating voltage is used.
Various types of monochromators have been proposed. Generally, a monochromator is composed of an energy filter portion for spectrally dispersing an electron beam and projecting a spectrum and an energy selector portion for extracting a desired energy width of electron beam from the spectrum and monochromatizing the beam by making use of slits or the like.
The energy filter portion produces a deflecting field in the beam path of the electron beam to spectrally disperse the beam by making use of difference of the orbit depending on electron velocity in the deflecting field, and projects a spectrum. The dispersive power of the energy filter portion is approximately tens of μm/eV.
The energy selector portion needs to have slits which are designed taking account of both the required energy width of the monochromatized electron beam and the dispersive power of the energy filter portion. The slits are from microns to submicrons in width. The slits are grooves formed, for example, in a thin metal film. Plural slits which are different in width are formed in the thin metal film to permit one to select energy widths of the electron beam under plural modes of operation according to observation conditions of the microscope (see, for example, JPA-2003-331764). The thin metal film can be positionally controlled via a mechanical position moving mechanism. All the slit widths can be installed in the electron beam path. An electron beam of some energy is cut off by the slits. An electron beam of other energy passes through the slits. As a result, the electron beam is monochromatized according to the widths of the slits.
In the case of the aforementioned monochromator, when an electron beam has passed through a slit, it is impossible to directly confirm the shape of this slit from the beam. Therefore, a human operator cannot directly confirm what slit width is located in the beam path.
Therefore, where an image is observed at a different energy width after switching the slit width, the operator must mechanically move the thin metal film, count the number of actions of switching the slit width on his/her fingers, and check what slit width is located in the beam path. In this way, the monochromator has been cumbersome to handle for unskilled operators.